Actually, the photon is its own antiparticle. A photon and an antiphoton are exactly the same thing. Further, the concept of time really has no meaning for a photon, since it always travels the speed of light.

Also, a negative velocity has the same meaning as a positive velocity -- it just means the object is moving in the opposite direction.

Negative energy could account for the attraction of masses by carrying momentum in the opposite direction to which it is moving and also it could be causing the acceleration of the universe because negative energy would repel positive masses whereas normal positive energy would deccelerate masses.
For more on this issue go to theory development - the mechanism of gravity .

I'm not really sure I understand lurch; I've never really heard of any "negative energy" particle.

- Warren

I haven't either, that's what I find so strange. Negative energy is known to exist, and I've done a little bit of research into it (though not very much, I'm afraid). It is only in the context of this discussion that I came to realize that I have never heard of a negative energy particle. In fact, I have never heard of negative energy being discussed in any quantized form.

You know, it's never occurred to me before that I've never heard of a negative energy photon.

There are no negative energy photons because photon energies are differences in atomic energy states. The absolute values of atomic energy states have no significance in and of themselves, but their differences do. Since photons are only emitted in transitions from higher states to lower states, you are bound to get a positive photon energy. Of course, you can always adopt the point of view that absorbed photons (those that promote electrons from lower states to higher states) are actually negative energy photons that move backwards in time.

Is negative energy not considered to be quantized?

No, the energy levels of the hydrogen atom are En=(-13.6eV)/n2, where n=1,2,3,...

As I said, that's just a matter of convention, and we can just as easily define the states to have positive energies, but there's something appealing about having the zero of energy at n-->infinity. That way, we associate negative energies with bound states, and nonnegative energies with free states.

Maybe this is way too obvious to mention, but in case anyone forgets, the relativistic Schrodinger equation, as solved by Dirac, contains both positive and negative energy eigenvalues. When Dirac solve this, he still had fermions, but with these negative energies. This was the first indication of antiparticles that was verified later.

http://www.phys.ufl.edu/~korytov/phz5354/lecture_D06.pdf [Broken]

So yes, there can be particles with "negative" energies. It is interesting to note that in condensed matter, these antiparticles correspond to the "holes" in the fermi sea below the vacuum state. So these holes also have negative energies.

Okay, this is probably going to sound idiotic, but it's 3:30 in the morning, I'm tired, and my mind is mixed up in Nemesis Theory, Evolutionism, Anti-Particles, and a story, but... Photons are particles, correct? And it doesn't make much sense to me to be your own anti-particle; if you are, you can't be annihalated. So, isn't it possible, by some weird anomaly, that anti-photons are not oppositely charged to Photons, but, rather, have the opposite gravitational pull?

Nope.In particle physics whe have a clear distinction of "antiparticle" and we have a clear rule of what might actually happens to the quantum field operators in order that that particle coincides with its own antiparticle.

The photon is its own antiparticle,just like the [itex] \pi^{0} [/itex] meson.

I'm not really sure I understand lurch; I've never really heard of any "negative energy" particle.

- Warren

According to the famous analysis of the irreducible unitary representations of the Poincaré group by E.P.Wigner who published an article [1] in 1939,QFT can accomodate,basically for any value of spin and mass,negative energy particles.

1) the curvature of space - positive energy curves space one way, negative the other... one is well-like, one is saddle-like but I can't remember which way round it is;

2) Hawking radiation, in which a pair of positive and negative energy particles are created on the event horizon of a black hole and separated by its gravity - the negative energy particle is pulled into the hole and reduces its mass and the positive energy particle is expelled away from it. I believe this might be what the OP is referring to, since as I recall it should appear as if the positive particle has come from inside the black hole.

If this 'negative energy' particle is indeed travelling backwards in time, then if it were to be absorbed it would look, forward in time, exactly like a positive energy particle being emitted. This, I guess, demonstrates why the photon is its own antiparticle: a 'negative' photon being absorbed is indistinguishible from a 'positive' photon being emitted and vice versa. So as Tom said, you could talk in terms of emission events being absorption of negative energies and absorption events being the emission of negative energies - convention and everyday experience dictate otherwise.